Linacs are used to create high-energy beams of electrons and/or x-rays. These are useful in a wide range of circumstances, including for medical purposes. Such beams are harmful to tissue in their path, and therefore by careful collimation and control of the beam they can be used to deliver a radiation dose to (for example) a tumour in order to cause the tumour to stop growing or even die back.
If a significant amount of non-cancerous tissue receives a significant dose, this can cause side-effects. These are undesirable in principle, and will also limit the rate at which the tumour can be irradiated. This, in turn, reduces the efficiency of the treatment. For this reason, the dose applied to the patient is carefully controlled, and complex treatment plans are prepared in which the direction of irradiation, the collimation of the beam, and the beam intensity are all varied with time in an interrelated manner in order to build up a desired three-dimensional dose distribution in the patient. These treatment plans are prepared by a treatment planning computer which models the interaction of the beam and the patient.
This process requires that the properties of the beam be well characterised, and that the beam continues to conform to that characterisation over the long term. An international standard exists which lays down ranges of permissible parameters for the beam, but these ranges are relatively wide and nominally identical linear accelerators may produce beams that all fall within the standard but which have measurably different properties.
Thus, the properties of the beam produced by a specific linear accelerator will usually be characterised prior to bringing the linear accelerator into service, and those properties used to define the beam model employed by the treatment planning computer. To confirm that the beam is still behaving according to the model, routine checks are carried out over time. Typically, there is a brief daily check, a slightly more thorough weekly check, and a more involved monthly check.